Fix for sys_munlock(2) with racct

Hi Edward, Alan,

I plan to commit the following patch:
http://people.freebsd.org/~jlh/racct_munlock.diff

This solves the following panic:

panic: racct_sub: freeing 301989888 of resource 5, which is more than allocated 73728 for pwsafe (pid 4177)

The problem is that the racct code in sys_munlock() trusts too much the
user's input.  vm_map_unwire_count() now returns how much memory has
really been unwired.

Any objection?
-- 
Jeremie Le Hen

Scientists say the world is made up of Protons, Neutrons and Electrons.
They forgot to mention Morons.